In the past, the strongest commercially produced permanent magnets were made from sintered powders of SmCo.sub.5. Recently, even stronger magnets have been made based on the light rare earth elements, preferably neodymium and praseodymium, iron and boron. These magnets contain a RE.sub.2 Fe.sub.14 B phase. These magnetic compositions and methods of processing them to make magnets are described in U.S. Pat. No. 4,496,395; U.S. Ser. Nos. 414,936 (filed 9/3/82); 508,266 (filed 6/24/83) now abandoned; and 544,728 (filed 10/26/83) to Croat; 520,170 (filed 8/4/83) to Lee; and 492,629 (filed 5/9/83) to Croat and Lee, all assigned to General Motors Corporation.
The rare earth (RE) elements include atomic numbers 57 to 71 of the Periodic Chart as well as yttrium, atomic number 39. Important sources of the rare earths are bastnaesite and monazite ores. Mixtures of the rare earths can be extracted from the ores by several well known beneficiating techniques. The rare earths can then be separated from one another by such conventional processes as elution and liquid-liquid extraction.
Once the rare earth metals are separated from one another, they must be reduced from their compounds to the respective metals in relatively pure form (95 atomic percent or purer depending on the contaminants) to be useful for permanent magnets. In the past, this final reduction was both complicated and expensive, adding substantially to the cost of rare earth metals.
The first reduction of rare earth halides was accomplished by their reaction with more electropositive metals such as calcium, sodium, lithium and potassium. However, the rare earth metals have a great affinity for such elements as oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphorous and hydrogen. Thus the reduced metals so produced were highly contaminated with very stable compounds of the rare earths and these elements. The yields of these reactions were also very low (about 25 percent) and the metal existed as small nuggets surrounded by alkali chloride slag. A discussion of early rare earth chloride reduction appears at pages 846-850, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed., Volume 19, 1982.
Today, both electrolytic and metallothermic (non-electrolytic) processes are employed to commercially reduce rare earth compounds to rare earth metals pure enough for use in industry. The electrolytic processes include (1) decomposition of anhydrous rare earth chlorides dissolved in molten alkali or alkaline earth salts, and (2) decomposition of rare earth oxides dissolved in molten rare earth fluoride salts.
Disadvantages of both electrolytic processes include the use of expensive electrodes which are eventually consumed, the use of anhydrous chloride or fluoride salts to prevent the formation of undesirable RE-oxy salts (NdOCl, e.g.), high temperature cell operation (generally greater than 1000.degree. C.), low current efficiencies resulting in high power costs, low yield of metal from the rare earth salt (generally 40 percent or less of the metal in the salt can be recovered). The RE-fluoride reduction process requires careful control of a temperature gradient in the electrolytic salt cell to cause solidification of rare earth metal nodules. An advantage of electrolytic processes is that they can be made to run continuously if provision is made to tap the reduced metal and to refortify the salt bath.
The most common metallothermic (non-electrolytic) processes are (1) reduction of RE-fluorides with calcium metal (the calciothermic process), and (2) reduction-diffusion of RE-oxide with calcium hydride or calcium metal. Disadvantages are that both are batch processes, they must be conducted in a non-oxidizing atmosphere, and they are energy intensive. In the case of reduction-diffusion, the product is a powder which must be washed repeatedly to purify it before use. Both processes involve many steps. One advantage of metallothermic reduction is that the yield of metal from the oxide or fluoride is generally better than 90 percent. Neither of these metallothermic reduction processes showed much promise for reducing the cost or increasing the availability of magnet-grade rare earth metals.
U.S. Ser. Nos. 627,736 (now abandoned) and U.S. Pat. No. 4,578,242, issued Mar. 25, 1986 both to Sharma filed July 3, 1984 and also assigned to General Motors Corporation are incorporated herein by reference. These applications relate to new, high-yield methods of metallothermically reducing rare earth oxides. However, in some circumstances it may be preferable to use RE chloride as feedstock for a rare earth reduction process. Therefore, the principal object of this invention is the creation of an improved method of metallothermically reducing rare earth chlorides.